The present invention relates generally to a preliminary thrusting mechanism for a screw device including a screw shaft and a nut assembly composed of two nuts operatively connected to each other in series and threadably engaged with the screw shaft wherein one of the screw shaft and the nut assembly is reciprocably displaced relative to the other one by rotating the screw shaft. More particularly, the present invention relates to improvement of a preliminary thrusting mechanism of the foregoing type wherein a certain intensity of preliminary thrusting power is imparted to two nuts in the opposite direction in such a manner as to allow the two nuts to be axially displaced away from each other.
To eliminate or minimize a backlash of the screw device, and moreover, reduce a quantity of elastic displacement of the screw device caused by a load imparted to the latter in the axial direction, i.e., improve rigidity of the screw device, a certain intensity of thrusting power is imparted to the screw device. Especially, in the case of a ball screw device including a number of balls interposed between a screw shaft and two nuts operatively connected to each other in series, a preliminary thrusting mechanism is usually classified into two types, one of them being a constant position type preliminary thrusting mechanism and the other one being a constant pressure type preliminary thrusting mechanism. As is well known for any expert in the art, the constant position type preliminary thrusting mechanism is generally constructed such that a preliminary thrusting power is imparted to the ball screw device by disposing a distance piece having a predetermined thickness between the two ball nuts and holding the two ball nuts while imparting a certain intensity of thrusting power to them in such a manner as to displace them toward or away from each other. On the other hand, the constant pressure type preliminary thrusting mechanism is constructed such that a thrusting member having a spring, liquid pressure or the like used therefor is disposed between two ball nuts and a constant intensity of thrusting power is imparted to the two ball nuts in the opposite direction so as to allow the two ball nuts to be displaced away from each other in order to achieve preliminary thrusting therewith.
In addition, a preliminary thrusting power variable type ball screw device is also known as disclosed in an official gazette of, e.g., Japanese Patent Laid-Open Publication No. 47-31050. According to this prior invention, the ball screw device is constructed such that an annular cavity is formed on the end surface of one of two ball nuts, an annular protuberance adapted to be fitted into the annular recess is disposed on the other ball nut, and a certain intensity of liquid pressure is supplied to the annular recess so as to thrust the one ball nut away from the other ball nut, causing both the ball nuts to be preliminarily thrusted with the liquid pressure. With the ball screw device constructed in the above-described manner, the liquid pressure and the preliminary thrusting power are normally held in the corresponding relationship.
However, in the case of the constant position type preliminary thrusting mechanism, since the screw device is rarely elastically deformed when-an axial load is imparted to it, causing it to exhibit high rigidity, it is practically used for performing a feeding operation at a comparatively low speed with high rigidity required therefor. However, when it is used for performing a feeding operation under a condition of a comparatively high speed without any necessity for maintaining high rigidity, there arises a malfunction that heat is generated from the screw device during the feeding operation at a high speed because the preliminary thrusting power is unnecessarily imparted to both the ball nuts. In view of the foregoing malfunction, when an intensity of preliminary thrusting power is set to be small in order to avoid the heat generation during the feeding operation at a high speed, it is certain that heat is rarely generated during the feeding operation at a high speed. However, there arises another malfunction that the ball device exhibits shortage of rigidity during the feeding operation at a low speed.
In the case of the constant pressure type preliminary thrusting mechanism, since the screw device exhibits low rigidity compared with the constant position type preliminary thrusting mechanism, there arises a malfunction in that both the nuts are deformed when a high magnitude of load is imparted to the screw device with high rigidity required for the latter.
In addition, in the case that both the nuts are preliminarily thrusted by the liquid pressure, since the preliminary thrusting power is imparted to both the nuts in proportion to the liquid pressure, there arises another malfunction in that the nuts are deformed due to the low rigidity of the screw device in the same manner as the constant pressure preliminary thrusting mechanism when a load having a magnitude larger than that of the preliminary thrusting load is applied to the screw device. Since the preliminary thrusting power is variable, it can be preset in the form of a preliminary thrusting power which preliminarily corresponds to a prospective load to be applied to the screw device. However, in the case that a certain intensity of preliminary thrusting power in excess of the preliminary thrusting load is imparted to the screw device at normal use, there arises another malfunction in that the screw device exhibit shortage of rigidity when a certain magnitude of load is rapidly applied to both the ball nuts or a high intensity of vibrative power is imparted to both the ball nuts. In addition, another malfunction is that the ball screw device exhibits low rigidity compared with the constant position type preliminary thrusting mechanism when the applied preliminary thrusting power remains within the range defined by the preliminary thrusting load.